The present invention relates to a projection exposure apparatus and a pattern forming method for use therewith. More particularly, the invention relates to a projection exposure apparatus capable of forming a fine pattern at a high level of contrast and to a profound depth of focus as well as to a pattern forming method for use with that apparatus.
Today, circuit patterns of solid state devices such as LSI's are getting ever finer in order to enhance the degree of their integration and the speed of their operation. The so-called step and repeat exposure method is generally used to form such fine patterns because of its capabilities to permit mass production and to ensure high levels of resolution. The resolution limit of that method is in direct proportion to exposure wavelength and in inverse proportion to the numerical aperture (NA) of the projection lens. Thus attempts have been made to shorten the exposure wavelength and to enlarge the numerical aperture for improved resolution limits. The depth of focus of the step and repeat exposure method is in direct proportion to exposure wavelength and in inverse proportion to numerical aperture squared. It follows that the depth of focus is getting considerably shallow even as the resolution limit is improving.
One conventional way proposed to enhance dramatically the resolution of the step and repeat exposure method is what is known as the phase-shifting method by which the phase of the light passing through an adjacent aperture on the mask is inverted. The phase-shifting method is discussed illustratively in "IEEE Trans. Electron Devices, Vol. ED-29, pp. 1828-1836 (1982)."
The so-called annular illumination method is a long-known method by which to improve the resolution of microscopes and like apparatuses. An application of the annular illumination method to optical lithography is described illustratively in "Digest of Papers, 1991 4th Microprocess Conference, pp. 70-71 (1991)."
Another conventional way to change the imaging characteristics of optical systems is optical filtering. This technique involves altering the phase-amplitude transmission distribution of the lens pupil. How the optical filtering may be applied to optical lithography is discussed illustratively in "Journal of Vacuum Science and Technology, Vol. B9, No. 6 (1991)."
In addition, Japanese Patent Laid-Open No. SHO/56-12615 proposes a combination of the annular illumination method with the optical filtering for attaining higher levels of resolution.
However, recent trends toward ever-higher integration of LSI's accompanied by the need for increasingly finer circuit patterns have reached a point where the unit circuit size is virtually equal to the wavelength of light. It has thus become difficult to enhance resolution by relying on the prior art attempts simply to enlarge numerical aperture or to shorten exposure wavelength. Moreover, a recent development involves making the electronic device structure three-dimensional for such representative LSI's as DRAM's. This means the LSI substrate surface on which to project a mask pattern is often left out of the shallow depth of focus. As a result, forming a fine pattern over the entire LSI chip surface is getting increasingly difficult.
Nevertheless, the high productivity of the step and repeat exposure method, combined with its high reliability based on many years of technical accumulation, compels one to expect that the method will continue to be applied to areas where the unit circuit size drops below the wavelength of light. This requires ensuring a high level of resolution in conjunction with a sufficient depth of focus.
The above-mentioned phase-shifting method when employed enhances appreciably the resolution of periodic patterns such as LSI wiring patterns. Where the spatial coherency of illumination light is raised, the depth of focus is significantly improved as a consequence. But there are some disadvantages involved. For example, suitable phase distribution can be difficult depending on the pattern shape. Another disadvantage is that the proximity effect of complex patterns makes it impossible to acquire transfer patterns that precisely reflect the mask pattern. A further disadvantage is that the phase-shifting method is not effective enough when it comes to dealing with isolated patterns such as hole patterns.
The annular illumination method when utilized enhances the resolution limit of periodic patterns but degrades the overall image contrast. In addition, the annular illumination method is not as effective as the phase-shifting method in terms of improving resolution.
The use of optical filtering deepens the depth of focus and improves the resolution limit where isolated patterns are to be formed. However, the optical filtering is not effective enough where periodic patterns are involved.
In the field of optical microscopes, there is proposed a combined use of the annular illumination method and the optical filtering for preventing the degradation of image contrast. However, it has never been clear what optimum conditions ought to be under which to apply the combination method to optical lithography. In particular, where patterns of diverse sizes are to be formed concurrently, the form of illumination and the filter shape need to be optimized in order to keep the contrast and light intensity uniform and to acquire a sufficiently profound depth of focus.
The combination of the annular illumination method with the optical filtering for application to optical lithography is discussed illustratively in "Extended Abstracts of the 52nd Autumn Meeting, 1991" of The Japan Society of Applied Physics (No. 2, p. 602). The combined use of the two techniques as discussed in the above reference requires that the form of an effective source be shaped in such a manner that the conjugate region of that source on the pupil will be located on the most peripheral part of the pupil, and that the amplitude transmission of the light passing through the most peripheral part of the pupil be reduced to 50 percent. Although this method improves the resolution at the focused position, the shallow depth of focus causes even a slight amount of defocusing to disrupt the pattern resolution. Furthermore, because the light intensity of images depends on the density of the mask pattern, it is difficult to apply the combination method to actual LSI patterns. Since optical lithography requires, given patterns of diverse sizes, uniform contrast and light intensity as well as sufficient depth of focus, satisfactory results are difficult to attain by use of the annular illumination method and optical filtering in combination.